GB1604205A - Method of manufacturing a unitary magnetic device - Google Patents

Description

PATENT SPECIFICATION

( 11) 1 604 205 ( 21) ( 62) Application No 26889/80 ( 22) Filed 3 May 1978 Divided out of No 1604204 ( 19) ( 31) Convention Application No 793394 ( 32) Filed 3 May 1977 in ( 33) United States of America (US) ( 44) Complete Specificaton Published 2 Dec 1981 ( 51) INT CL 3 Gl C 11/04 ( 52) Index at Acceptance H 3 B 200 208 236 237 271 ED B 3 A 44 B 3 E 14 L 6 A 8 AK ( 54) A METHOD OF MANUFACTURING A UNITARY MAGNETIC DEVICE ( 71) We, THE ECHLIN MANUFACTURING COMPANY, a Corporation organised under the Laws of the State of Connecticut, United States of America, of Echlin Road, and U S 1 Branford, Connecticut, United States of America, do hereby declare the invention for which we pray that a patent may be granted, and the method by which it is to be performed to be particularly described in and by the follow-

ing statement.

This application is divided from British Patent Application No 17495/78 Serial No.

1604204 filed 3rd May, 1978, and relating a magnetic device having two magnetic states, a reverse state in which core and shell have opposite directions of magnetisation and a confluent state in which core and shell have the same direction of magnetisation.

That application has a main claim reading, "A magnetic device having first and second magnet portions with the same chemical alloy composition and of elongate form extending contiguously, the net coercivity of the first portion being substantially greater than that of the second portion, and the respective dimensions and coercivities of the portions being such that the device has a confluent state into which it can be put by an external field, in which confluent state the first and second portions have the same direction of magnetisation, and a reverse state wherein the low coercivity second portion forms a return path for remanent flux of the high coercivity first portion, in which reverse state the portions are separated solely by a magnetic interface, characterised in that the second portion has a coercivity sufficiently great to ensure that when the device is in the confluent state, the remanent magnetisation of the first portion is inadequate to switch the device into the reverse state, and an external field is required to do so " The 'coercivity' is the coercive force required to bring the flux density of magnetised material to zero.

The present invention relates to a method of making a magnetic device, for example, a device as claimed in that claim.

According to the present invention, a method of making a unitary magnetic device comprising the steps of:

holding a length of wire under tension; and apply cycling torsional strain to said wire under tension, the net torsional strain in one direction being substantially greater than the net torsional strain in the other direction, and elongating the wire, for example by between one and two percent, during the application of torsional strain.

A preferred composition for preparing the wire is an alloy of iron and cobalt and vanadium In one embodiment, the wire is 52 percent cobalt, 10 percent vanadium and the balance iron The vanadium appears to enhance coercivity without decreasing the ductility required for cold working.

In one embodiment of the invention a wire approximately one quarter millimeter ( 0.010 inches) in diameter and about 30 centimetres ( 12 inches) in length is subjected to sufficient tension to straighten it out but without stretching the wire Thereafter, the wire is subjected to torsional straining combined with elongation The torsional straining is in alternate clockwise and counterclockwise directions and provides the necessary work hardening for the wire The torsional straining schedule is asymmetrical; that is, the number of turns given in each direction is not the same At the end of the cold working routine the wire is given an ageing heat treatment by having a substantial pulse of current passed through it.

The result is a switching device which has the significant characteristic that in the absence of an external field it will not tn m I 1 604 205 automatically switch from the state in which core and shell have the same direction of magnetisation (confluent state) to the state where the core and shell have opposite direction of magnetisation (reverse state).

Furthermore, the result is a switching device having an asymmetric switching characteristic That is, the induced pulse when the direction of magnetisation of the core ) switches in a first direction, relative to the shell, is different from the induced pulse when the core switches in a second direction relative to the shell More specifically, when the device switches from the reverse state to the confluent state the induced pulse is substantially greater than is the pulse induced when switching from the confluent state to the reverse state.

The magnetic switching device thus proD vided when switching from reverse state to confluent state does so with a large rate of change of flux and thus with a larger output pulse from a pick-up coil The device is relatively insensitive to ambient conditions, including most ambient magnetic fields, and thus is useful in timing, proximity detection and coding applications.

The invention may be carried into practice in various ways, and certain embodi0 ments will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 is an enlarged diagrammatic representation including a longitudinal view and an end view, of a ferro-magnetic wire manufactured by the method of the present invention Figure 1 represents the magnetisation of shell and core in the "reverse" state where shell and core magnetisation are OD in opposite directions; and Figure 2 is a perspective schematic representation of the technique of processing the magnetic wire.

The wire is to be used in segments of l 5 about one to three cm When magnetised, each wire segment has two magnetic states.

When switching between these two magnetic states, at least a portion of the flux switches direction so that a pick-up coil i O wound around the wire will generate a pulse The rate at which the flux switches when the wire changes state is so fast that the electrical pulse generated by the pick-up coil is a distinctive, sharp, usable pulse approximately 20 microseconds in duration.

The switch in state occurs in response to an external magnetic field, having a proper direction, either increasing in magnetic field intensity to above a first threshold or decreasing in magnetic field intensity to below a second threshold The switching of the wire, thus, is responsive to a threshold magnetic field applied to the wire As a result, the magnitude of the output pulse is essentially not rate sensitive in thatit is only slightly affected by the rate at which the external triggering magnetic field increases or decreases; at least this is the case up to very high rates of field change The use of this wire for generating this distinctive, high consistent, output pulse has the further advantage that the process occurs without requiring any input electrical signal or current Thus external permanent magnets can be used as source of the triggering magnetic field and all that is required is that the position between the bistable magnetic wire and the external permanent magnets be changed to provide the increase of external field over the first threshold and/or the decrease of external field under the second threshold Even where the triggering magnetic field is generated by an electric current through a coil around the wire, there is no need for other electrical inputs at the switching device.

It is believed that this bistable magnetic wire operates as it does because of the intimate physical relationship between a magnetically harder shell zone and a magnetically softer core zone This intimate physical relationship is due to the fact that both shell and core are elements of an otherwise homogeneous wire The mechanism by which this new phenomenon operates is still being investigated.

With reference to Figure 1, magnetic wire comprises a work hardened magnetic material composed of cobalt, iron and vanadium The magnetic wire segment has a generally circular cross section, preferably a true around cross section or as close to true round as can be reasonably obtained Wire segments about 0 25 millimeters in diameter and one to three centimeters in length have been found useful.

The wire is processed, as described below, to provide a unitary magnetic wire element 10 having a relatively "soft" core 11 having relatively low magnetic coercivity and relatively "hard" shell 12 having relatively high magnetic coercivity.

The term "coercivity" is used herein in its traditional sense to indicate the magnitude of the external magnetic field necessary to bring the net magnetisation of the magnetised sample of ferromagnetic material to zero.

With reference to Figure 1, the relatively "soft" core 11 is magnetically anisotropic with an easy axis of magnetisation substantially parallel to the axis of the wire The relatively "hard" shell is also magnetically anisotropic with an easy axis of magnetisation providing a net magnetisation substantially parallel to the axis of the wire The direction of magnetisation of the core 11 is in large part a function of the interaction of the magnetic field of the shell and whatever external magnetic field is applied In the

1 604 205 state shown in Figure 1, the net magnetisation of the core 11 is opposite in direction from the net magnetisation of the shell 12.

This state is referred to herein as the reverse state In this reverse state, a domain wall interface 13 defines the boundary between core 11 and shell 12 This interface 13 is shown in Figure 1 as a cylindrically shaped boundary wall 13, although it is believed that the domain wall interface occurs as a rather complex magnetic transition zone in the wire.

It has been found that pulses may be obtained from wire composed of cobalt, iron and vanadium which are at least one order of magnitude greater than the pulses obtained from nickel-iron alloy wire.

A preferred composition for the wire is one in which the content of cobalt is from about 45 to 55 percent, the content of iron is from about 30 to 50 percent, and the content of vanadium is between about 4 and 14 percent A commercially available alloy of cobalt, iron and vanadium of 0 25 millimetres diameter has been found suitable for practising the present invention; one such wire is available from Wilbur B Driver Co., Inc, under the Trade Name "VICALLOY" Vicalloy wire has a composition, nominally, of about 52 percent cobalt, about percent vanadium, and the remainder substantially iron with certain minor constituents including manganese and silicon in amounts slightly under one-half of one percent each.

Figure 2 is a mechanical schematic to represent one mechanism used in cold working the wire A length of wire 40, for example 30 cm is pulled off a spring loaded reel 46 Tension is thereby held on the wire to keep it straight The wire 40 is fed through chuck 42 to chuck 44 The chucks 42, 44 are then tightened to hold the wire in place Cyclical torsional straining of the wire 40 is then effected by alternate rotation of the pinion 48 on the rack 50 The rack 50 moves back and forth because it is eccentrically mounted on the plate 52 which is driven by the motor 54 Elongation of the wire 40 is effected by slow rotation of the cam 56 which bears against the ear 58 on the chuck 42 The cam 56 is rotated by motor drive 60.

First Treatment Schedule Using a 30 centimeter length of this Vicalloy alloy having a diameter of one quarter millimeter ( 10 mils), a preferred work hardening schedule consists of the following steps; First The wire is stretched out to its full length As shown in Figure 2, the length of wire 40 is secured in chucks 42 and 44.

Enough tension is applied to the wire through a spring loaded reel 46 to hold the wire 40 at its unbent or uncurled length, without elongating the wire The wire 40 is then subjected to a single cycle of torsional strain comprising approximately 64 counterclockwise turns followed by approximately 48 clockwise turns The tension is maintained during all torsional straining steps.

Second The wire is then subjected to seventeen and one-half cycles of eight and one-half turns in each direction More specifically, 8-1/2 counterclockwise turns followed by 8-1/2 clockwise turns are applied and constitute one cycle The cycle is repeated sixteen times and then this second step is completed with 8-1/2 counterclockwise turns During this second step, which normally lasts about 10 to 15 seconds, the 30 centimeter wire is continuously slowly elongated; the amount of elongation being between one percent and two percent.

Third The final step in the work hardening schedule consists of another series of eight and one-half turns, this time for an even number of cycles, and without further stretching but maintaining tension on the wire Three to four times the number of cycles used in the second step are employed during this step About 60 cycles have been found to give good results.

The wire is then cut into usable segments of, for example, 1 to 3 cm lengths.

The schedule described above for work hardening an iron, cobalt, vanadium alloy wire, has been found to give certain desired results In determining these desired results, it was also found that variations in the above schedule result in a wire which still exhibits the switching effect.

Second Treatment Schedule A less preferred wire treatment schedule that has been found effective with this Vicalloy wire for those applications where maximum time stability is not important is as follows A 30 centimeter length of one quarter millimeter diameter is used.

First The wire is stretched out to its full length The tension applied holds the wire straight at its full length, without elongating the wire The wire is then subjected to a single cycle torsional strain comprising 14 counterclockwise turns followed by 12 clockwise turns.

Second The wire is then subjected to 120 cycles of twelve turns in each direction.

More specifically, 12 counterclockwise turns followed by 12 clockwise turns are applied and constitute one cycle This cycle is repeated 120 times During this second step of the work hardening schedule, the wire is continuously stretched during the torsional straining action During this second step, the 30 centimeter wire is elongated slowly and continuously by about three millimeters.

Third The final step in the work harden1 604 205 ing schedule consists of another series of twenty cycles of twelve counter-clockwise and twelve clockwise turns without further elongation but maintaining tension on the wire so that the elongation imposed in step two is maintained.

The wire is then cut into desired lengths of, for example, 1 to 3 centimeters.

The alloy used in both work hardening schedules is essentially the same It is initially annealed to assure a uniform starting material and to assure adequate ductility for the work hardening schedule The wire is preferably initially annealed to the point where the grain structure is approximately 10,000 grains (or more) per square millimeter This fine grain structure aids in ensuring the required ductility.

A fourth step has been found important in connection with both of the schedules mentioned above This fourth step is a heat treatment step During the early stages of experimentation, this heat treatment was at approximately 320 C for approximately i eight hours However, it was found satisfactory to run the heat treatment step for four hours at approximately 300 C and such had the benefit of speeding up the processing of the wire It is presently preferred to perform ) the heat treatment step by sending a 5 6 ampere current through this 0 25 mm wire for 120 milli-seconds The heat treatment produces a discernable improvement in the output pulse Perhaps more importantly, this heat treatment reduces the risk that the characteristics of the wire will change in use as a wire is subjected to a high temperature environment This fourth step of post-workhardening heat treatment provides an ) ageing which resuls in stability in use.

Claims (8)

WHAT WE CLAIM IS:

1 A method of manufacturing a unitary magnetic device comprising the steps of; holding a length of wire under tension; and applying cycling torsional strain to said wire under tension, the net torsional strain in one direction being substantially greater than the net torsional strain in the other 0 direction, and elongating the wire during the application of torsional strain.

2 A method as claimed in Claim 1 further comprising the step of annealing said wire prior to said step of applying cycling torsional strain.

3 A method as claimed in Claim 1 or Claim 2, further comprising the step of heat treating said wire after said step of applying cycling torsional strain.

0

4 A method as claimed in any of Claims 1-3 in which the wire is elongated by approximately between one and two percent during said step of applying torsional strain.

A method as claimed in any of Claims 1-4 wherein said step of applying torsional strain includes between 30 and 120 cycles of approximately 8 to 12 turns per 30 centimeters of length.

6 A method as claimed in any of Claims 1-5 wherein during said step of applying torsional strain, said wire is elongated for a portion of said step and the length of said wire is held constant for another portion of said step.

7 A method as claimed in any of Claims 1-6 in which the strain is effected by at least one cycle of asymmetric torsional strain to said wire, the magnitude of the strain applied being at least two turns per centimeter, and cycles of symmetric torsional strain, said symmetric strain being substantially less than one turn per centimeter, the number of symmetric cycles being substantially greater than the number of cycles of asymmetric straining.

8 A method as claimed in any of Claims 1-7 in which the wire has an average size of about 10,000 grains per square millimeter.

KILBURN & STRODE, Chartered Patent Agents, Agents for the Applicants.

Printed for Her Majesty's Stationery Office.

by Croydon Printing Company Limited Croydon Surrey 1981.

Published by The Patent Office 25 Southampton Buildings.

London, WC 2 A IAY, from which copies may be obtained.